In photolithography, masks are used to expose a pattern upon a semiconductor wafer for the formation of integrated circuits and structures. As manufacturing requirements call for exposure of patterns with smaller and smaller dimensions, it is becoming necessary to employ techniques which permit enhancement of the current performance of the process of photolithography.
A reduction projection exposure method that features mass-producibility and excellent resolution has been used widely for forming such patterns. According to this method, the resolution varies in proportion to the exposure wavelength and varies in inverse proportion to the numerical aperture (NA) of the projection optical system. The NA is a measure of a lens' capability to collect diffracted light from a mask and project it onto the wafer. The resolution limit R (nm) in a photolithography technique using a reduction exposure method is described by the following equation: EQU R=Kl.times..lambda./(NA)
Where: .lambda. is the wavelength (nm) of the exposure light
NA is the numerical aperture of the lens PA2 Kl is a constant dependent on a type of resist
So far, increases in the resolution limit have been achieved by increasing the numerical aperture (high NA). This method, however, is approaching its limit due to a decrease in the depth of focus and difficulty in the design of lenses and in the lens fabrication technology itself. In recent years, therefore, attention has been given to the approach for shortening the wavelength of the exposure light in order to form finer patterns to support an increase in the integration density of LSIs. For example, a 1-Gbit DRAM requires a 0.2-micrometer pattern while a 4-Gbit DRAM requires a 0.1-micrometer pattern. In order to realize these patterns, exposure light having shorter wavelengths must be used.
Typically, optical photolithography is achieved by projecting or transmitting light through a pattern made of optically opaque areas and optically clear areas on a mask. The optically opaque areas of the pattern block the light, thereby casting shadows and creating dark areas, while the optically clear areas allow the light to pass, thereby creating light areas. Radiation is projected through the optically clear areas onto and through a lens and subsequently onto a substrate.
However, because of increased semiconductor device complexity which results in increased pattern complexity, and increased pattern packing density on the mask, distance between any two opaque areas has decreased. By decreasing the distances between the opaque areas, small apertures are formed which diffract the light that passes through the apertures. The diffracted light results in effects that tend to spread or to bend the light as it passes so that the space between the two opaque areas is not resolved, therefore, making diffraction a severe limiting factor for optical photolithography.
A conventional method of dealing with diffraction effects in optical photolithography is achieved by using a phase shift mask, which replaces the previously discussed mask. Generally, with light being thought of as a wave, phase shifting is a change in timing of a shift in wave form of a regular sinusoidal pattern of light waves that propagate through a transparent material.
Typically, phase-shifting is achieved by passing light through areas of a transparent material of either differing thicknesses or through materials with different refractive indexes, or both, thereby changing the phase or the periodic pattern of the light wave. Phase shift masks reduce diffraction effects by combining both diffracted light and phase shifted diffracted light so that constructive and destructive interference takes place favorably. On the average, a minimum width of a pattern resolved by using a phase shifting mask is about half the width of a pattern resolved by using an ordinary mask.
Nonetheless, an inherent problem with a conventional transmission mask, such as the ones described above, is that the substrate undergoes a decrease in transmissivity as the wavelength of light emitted from an exposure light source is decreased to obtain finer patterns. For example, a quartz material substrate becomes more opaque as the wavelength of the light source decreases, particularly when the wavelength is less than 200 nm. This decrease in transmissivity affects the ability to obtain finer resolution patterns. For this reason, a material for a transmission phase shifting mask which can obtain a high transmissivity with respect to light having a short wavelength is needed. It is, however, difficult to find or manufacture such a material having a high transmissivity with respect to short wavelength exposure light.
Under these circumstances, a reflective mask has recently been proposed. In a reflective mask, a recess portion consisting of a reflective phase shifting material is formed on a substrate surface for producing a high resolution pattern while avoiding the problem of a deterioration in quality when the wavelength of light emitted from an exposure light source is decreased. The recess portion is formed by etching, such that the substrate is non-planar, thus allowing the reflective material to fill in the recess areas.
A reflective material formed on a substrate surface has a particular refractive index. As previously stated, a recess or nonplanar portion is formed on the substrate surface consisting of the reflective phase shifting material. A phase difference is obtained by using an optical path difference between the light reflected by the substrate surface and the light reflected by the phase shifting material. Depending on the selection and arrangement of the reflective material and the depth of the recess, different phase shifts may be induced.
In contrast to a transmission mask, not all of the light emitted from an exposure light source goes through a reflective mask because the substrate tends to become opaque as the wavelength of the light source decreases, i.e., less than 200 nm. In fact, most of the light will be blocked by the mask's substrate.
The following problem is posed in a mask of this type. The recess portion is formed by etching. However, it is very difficult to accurately control the etching depth. When the reflection of light is utilized, the amount of change in optical path length is equivalent to twice the amount of change in etching depth. It is, therefore, substantially difficult to realize a controlled phase difference between light reflected by the substrate surface and light reflected by the recess portion.
In summary, reflective masks have been proposed to overcome an inherent problem of transmission phase shifting masks, which is the deterioration in quality at the shorter wavelengths required to form finer line widths. The reduction in transmissivity of the substrate results in a limited achievable resolution of a mask pattern being exposed upon a semiconductor wafer for the formation of integrated circuits and structures. However, a drawback of a reflective mask is the performance of the mask being dependent upon the optical path length; which is effected by both the height of the reflective phase shifting material and the recess depth etched into the substrate. It is difficult to accurately control the etching depth, thus a critical factor is presented.
Hence, there is a need for a reflective mask wherein the optical properties of the mask are independent of the thickness of the reflecting materials, and the optical properties are also independent of the depth of the recess portions formed on a substrate.